This invention relates in general to infrared detectors and more particularly to a microbolometer and the method for forming the same.
Infrared (IR) detectors are often utilized to detect fires, overheating machinery, planes, vehicles, people, and any other objects that emit thermal radiation. Infrared detectors are unaffected by ambient light conditions or particulate matter in the air such as smoke or fog. Thus, infrared detectors have potential use in night vision and when poor vision conditions exist, such as when normal vision is obscured by smoke or fog. IR detectors are also used in non-imaging applications such as radiometers, gas detectors, and other IR sensors.
Infrared detectors generally operate by detecting the differences in thermal radiance of various objects in a scene. That difference is converted into an electrical signal which is then processed. Microbolometers are infrared radiation detectors that are fabricated on a substrate material using traditional integrated circuit fabrication techniques. After fabrication, microbolometers are generally placed in vacuum packages to provide an optimal environment for the sensing device. Conventional microbolometers measure the change in resistance of a detector element after the microbolometer is exposed to thermal radiation. Microbolometers have applications in gas detectors, night vision, and many other situations.
The primary factors affecting response time and sensitivity of microbolometers are thermal mass and thermal isolation. Microbolometer response time is the time necessary for a detector element to absorb sufficient infrared radiation to alter an electrical property, such as resistance, of the detector element and to dissipate the heat resulting from the absorption of the infrared radiation. Microbolometer sensitivity is determined by the amount of infrared radiation required to cause a sufficient change in an electrical property of the microbolometer detector. Microbolometer response time is inversely proportional to both thermal mass and thermal isolation. Thus, as thermal mass increases, response time becomes slower since more infrared energy is needed to sufficiently heat the additional thermal mass in order to obtain a measurable change in an electrical property of the microbolometer detector element. As thermal isolation increases, response time becomes slower since a longer period of time is necessary to dissipate the heat resulting from the absorption of the infrared radiation. Microbolometer operating frequency is inversely proportional to response time. However, microbolometer sensitivity is proportional to thermal isolation. Therefore, if a specific application requires high sensitivity and does not require high operating frequency, the microbolometer would have maximum thermal isolation and minimal thermal mass. If an application requires a higher operating frequency, a faster microbolometer may be obtained by reducing the thermal isolation which will also result in a reduction in sensitivity.
In order to maximize the sensitivity of microbolometers, the temperature coefficient of resistance of the detector element in the microbolometer should be as high as possible.
From the foregoing, it may be appreciated that a need has arisen for an improved microbolometer and method for forming the same. In accordance with the present invention, a microbolometer and method for forming the same is provided which substantially eliminates or reduces the disadvantages and problems associated with conventional micro infrared detectors.
According to one embodiment of the present invention, there is provided a microbolometer and method for forming comprising an absorber element that changes temperature in response to absorbing infrared radiation and an amorphous silicon detector suspended above a silicon substrate at a height of one-quarter wave length of the infrared radiation to be detected. The amorphous silicon detector changes electrical resistance in response to the absorber element changing temperatures. The microbolometer further comprises electrode arms coupled to the silicon substrate providing structural support for the amorphous silicon detector and electrical connectivity for the microbolometer.
The technical advantages of the present invention include providing a microbolometer of substantially lower thermal mass than conventional microbolometers. The substantially lower thermal mass results in increased operating frequency and increased thermal isolation for the microbolometer. The increased thermal isolation results in increased sensitivity such that less infrared radiation is required to cause a detectable change in the electrical resistance of the microbolometer detector. Another technical advantage of the present invention includes a thermal shunt that may be varied during fabrication to obtain microbolometers with differing operating frequency and sensitivity characteristics. By increasing the thermal shunt material, the thermal coupling between the microbolometer and the substrate material is increased and the thermal isolation of the microbolometer is correspondingly decreased. This results in a microbolometer with an increased operating frequency and decreased sensitivity. Yet another technical advantage of the present invention is the use of spiral arms to minimize the area required for a given electrode arm length thereby maximizing the area available for the microbolometer detector element.
Other technical advantages will be readily apparent to one skilled in the art from the following figures, description, and claims.